Firewolf, what you're saying is kind of like saying "Why do I see SM lists with assault squads and veteran units? What's wrong with tactical marines?!"
Cult units are good, strong, specialized units with a fair price. If you want an indestructible fireteam to hold an objective, take some plague marines (they can also have 2 special weapons in a small squad). You want survivable anti-meq footsloggers, grab some sons. You want a killer mobile firebase, go for noiseys. You want some nasty mounted assault, get zerks.
Meanwhile, if you want a versitile, low-cost, unspecialized unit take some CSMs.
There's no reason not to take cult units these days. They're good specialists and the fluff has changed so that the original legions are broken and hired out as mercs, or so independent sorcerers can replicate their methods.
For better or worse, cult units are now just specialist squads that anyone can take. No reason to only work out of a fraction of your codex unless your personal fluff dictates it.